A mass spectrometry system is an analytical device that determines the molecular weight of chemical compounds by separating molecular ions according to their mass-to-charge ratio (m/z). Ions are generated by inducing either a loss or gain of charge and are then detected. Mass spectrometry systems generally comprise an ionization source for producing ions (i.e. electrospray ionization (EI), atmospheric photoionization (APPI), atmospheric chemical ionization (APCI), chemical ionization (CI), fast atom bombardment, matrix assisted laser desorption ionization (MALDI) etc.), a mass filter or analyzer (i.e. quadrupole, magnetic sector, time-of-flight, ion trap etc.) for separating and analyzing ions, and an ion detector such as an electron multiplier or scintillation counter for detecting and characterizing ions.
The first mass analyzers introduced in the early 1900's used magnetic fields for separating ions according to their mass-to-charge ratio. Just as ionization sources have evolved so have the mass analyzers to meet the demands of various chemical molecules. One type of mass analyzer is the ion trap. Ion trap mass analyzers operate by using two or more RF ring electrodes to trap ions of a particular mass-to-charge ratio. The ion trap mass analyzer was developed around the same time as the quadrupole mass analyzer and the physics behind both of these analyzers are very similar. These mass analyzers are relatively inexpensive, provide good accuracy and resolution, and may be used in tandem for improved separations. Typical mass range and resolution for ion trap mass analyzers are (Range m/z 2000; Resolution 1500). Other advantages of ion traps include small size, simple design, low cost, and ease of use for positive and negative ions. Ion trap mass analyzers have, therefore, become quite popular. However, ion traps suffer from a few particular problems. For instance, the limited range of current commercial versions as well as low energy collisions and ion fragmentation problems.
In order to address these problems MS/MS, 2-dimensional (2D) and 3-dimensional (3D) analytical techniques and methods of fragmentation have been developed. Commonly in 3D ion trap mass spectrometry the fragmentation is achieved by setting the main RF voltage to relatively high values to increase the depth of pseudo-potential trapping wells and also by applying a supplemental field on resonance with the fundamental frequency of the ion motion. The value of the RF amplitude that is used for the fragmentation can be expressed in terms of a dimensionless parameter q. Typically, q can range from 0 to 0.908 and from various derived equations the lowest stable mass within the ion trap can be determined. The lowest stable mass within the RF field is called the fragmentation cut-off limit. All the fragment ions with masses below the fragmentation cut-off limit are unstable within the RF ion trap and are impossible to analyze. Fragmentation cut-off has been an ongoing problem for ion traps and has limited the overall potential effectiveness and flexibility of ion traps.
It, therefore, would be desirable to alleviate this problem by substantially reducing the fragmentation cut-off for ion trap systems. In addition, it would be desirable to expand the range and types of molecules that may be analyzed using ion traps. For instance, it would be desirable to decrease fragmentation cut-off so low molecular weight fragmentation information can be used and developed for sequencing and characterizing various small molecules and peptides. In addition, it would be desirable to be able to isolate, trap and scan molecules of various sizes without having to move them between mass analyzers and/or collision cells. These and other problems presented have been obviated by the present invention.